1. Field of the Invention
The present invention relates to toner and liquid developer including the toner which are used for image formation, and particularly relates to electrophotographic toner and an electrophotographic liquid developer which are used for developing electrostatic latent images in image forming apparatuses such as electrophotographic copying machines and printers. More particularly, the invention relates to yellow toner and a yellow liquid developer.
2. Description of the Background Art
Offset printing has been employed as a method for formation or duplication of fine and beautiful full-color images. The offset printing allows fast production of high-quality images with a low cost, and therefore is suitable to large-volume printing. However, maintenance and operation of the offset printing apparatus are difficult.
With the increasing use of personal computers and others, it has been demanded that various kinds of full-color images can be flexibly formed or duplicated by simple operations.
In connection with this, electrophotographic copying machines, printers and others have such advantages that a printing cost per sheet does not substantially increase even if the sheets to be processed are small in number. Compared with other printers of an ink-jet type or the like, the electrophotographic device is superior in printing speed, reproducibility and durability.
In the electrophotographic image formation, the developing method is classified into a dry developing method and a wet developing method.
In the dry developing method, a developer is formed of toner and carrier which has magnetic properties or the like and is added to the toner. Usually, the dry toner includes, as major components, pigment and binder resin, and also includes, if necessary, a charge director, a conductivity control agent, a plasticizing agent, a releasing agent and others in a contained or added fashion. Magnetic toner includes magnetic powder of, e.g., magnetite (Fe.sub.3 O.sub.4). In the dry developing method, the toner is usually charged by the contact with a specific surface of the developing device, the mutual contact between the toner, contact charging as a result of contact with carrier or the like in the case of two-component developer including the carrier, electrostatic induction by an electric field, charge injection, ion absorption caused by ionizing discharge of air, or the like. The toner thus charged is transported onto an electrostatic latent image portion on an electrostatic latent image carrier such as a photosensitive member, e.g., by an electrostatic, mechanical and/or magnetic forces, and is used for development by an electrostatic force.
The dry toner used for dry development cannot be fine because excessively fine toner may flow into an ambient atmosphere and float therein. Therefore, the toner which is usually used has a relatively large average particle diameter of about 10 .mu.m. The dry toner having such a relatively large particle diameter can achieve high-density image without difficulty.
In the wet developing method, the liquid developer which is now in the mainstream is formed of electrically insulating carrier liquid, in which toner primarily made of a coloring agent and binder resin as well as a charge director, a dispersion stabilizing agent and others are dispersed. It has been considered that the toner is charged owing to absorption of ions by virtue of the charge director, and the charged toner is used for development on the principle of electrophoresis.
Since there is no possibility that the toner used in the wet development escapes into the atmosphere, fine toner can be used, and the available average particle diameter may be of the order of submicrons. Thereby, the produced image can have a high resolution and a good gray scale property.
For forming the full-color images, developers of four colors, i.e., yellow, magenta, cyan and black are used similarly to the offset printing and others, and images each formed with the developer of the corresponding color are overlapped with each other by a subtractive color mixture method. If these developers of four colors are of a dry type, each developer can be produced by adding appropriate coloring pigment or dye as well as various additives, if necessary, to binder resin in a dispersed fashion, and thereby providing toner of a predetermined particle diameter. In the case of the liquid developer, the developer can be produced by dispersing such toner in the carrier liquid.
Among the toners of respective colors, the yellow toner is primarily formed of Color Index Pigment Yellow (which will also be referred to as "PY" hereinafter) 12, PY13, PY17, PY174, PY176 or the like, which has been widely used in conventional printing ink and others, because these pigments are preferable in view of coloring properties and cost.
However, the above Pigment Yellow contains organically bonded chlorine and heavy metal in the molecule. Therefore, it is proposed to use PY180 which is an azo compound containing no organically bonded chlorine and heavy metal and has excellent color forming ability almost same as conventional Pigment Yellow. PY180 is an azo pigment which has been known under the trade names of Novoperum-Gelb P-HG, Toner Yellow HG VP2155 and others. PY180 and dry toner containing the same are disclosed in U.S. Pat. Nos. 4,870,164 and 4,935,502, European Patent Application No.0 705 886 A2, and others.
However, if the compound classified as PY180 and/or derivative thereof is used as a coloring agent in the toner for the dry development or a coloring agent in the toner of the liquid developer, the toner has a lower chargeability than toner using conventional yellow pigment, and therefore cannot achieve a practically required high developing speed. For achieving high-resolution images, it is necessary to reduce the toner particle diameter. In this case, the toner chargeability must be further increased for achieving the intended developing speed. However, the toner including the compound classified as PY180 and/or derivative thereof has the low chargeability, and therefore cannot provide the fine images.
If the compound classified as PY180 and/or derivative thereof is used as a coloring agent in the toner for the dry development or a coloring agent in the toner of the liquid developer, the dispersibility of pigment in the binder resin is lower than that in the case where the conventional yellow pigment is used, and therefore light transparency of final images is low. If the transparency of each color in the full-color image is not sufficient, the underlying color cannot be sufficiently seen through the upper color, resulting in low reproducibility of each color. If the toner containing PY180 and/or derivative thereof is used, spectral reflection characteristics are low because the pigment has a low dispersibility in the binder resin so that the color reproducibility is low when the colors are converted into numerical values by spectrocolorimetry.